1. Field of the Invention
The present invention generally relates to a wireless communication technique, and more particularly to a multiple antenna system comprising a plurality of antenna devices, each being provided in one of multiple sectors, and a base station apparatus connected to the antenna devices via optical cables.
2. Description of the Related Art
In mobile communications systems, the space in which communication services are provided is divided into multiple cells or sectors, and wireless communication is carried out between mobile terminals located in certain sectors (or cells).
FIG. 1 is a schematic diagram illustrating a part of a code division multiple access (CDMA) system, which is typically employed in the wireless communication field. Multiple antenna units (AU) 104 are connected to the base station apparatus 110 via associated signal converters 106 and optical cables 108. The base station apparatus 110 includes optical to electric converters 112, each being connected to one of the optical cables 108, and baseband processors 114 connected to the associated signal converters 112.
One of the antenna units 104 is provided in each sector to transmit and receive radio signals to and from a mobile terminal (not shown) located in the sector. When receiving signals, the antenna unit 104 converts the radio signals to digital signals. When transmitting signals, the antenna unit 104 converts digital signals to analog signals.
The signal converter 106 performs electric-to-optical conversion to convert electric signals (uplink signals) received at the antenna unit 104 and converted into the digital format, into optical signals. In addition, the signal converter 106 performs optical-to-electric conversion to convert optical signals (downlink signals) carrying information into electric signals.
The optical cable 108 transmits optical signals between the antenna unit 104 (to be more precise, the signal converter 106) and the base station apparatus 110. The optical cable 108 is capable of signal transmission with less transmission loss and less signal degradation because it allows optical signals to propagate through it.
The signal converter 112 converts the optical signals (uplink signals) to electric signals, and converts electric signals (downlink signal) carrying information into optical signals.
The baseband processor 114 carries out baseband processes, such as spread spectrum using a code for a transmission signal and despreading of a received signal. The baseband processor 114 communicates with a higher-layer apparatus or a core network (neither shown).
The above-described baseband apparatus is disclosed in, for example, JP 2002-335560A and JP 2002-118870A.
A mobile terminal moves across sectors in a service area, while continuing wireless communication, using handover. Signals input to and output from the antenna units 104 provided to the respective sectors have to be synchronized with each other. To this regard, the technique disclosed in JP 2002-335560A assumes that the quantity of delay is measured and adjusted manually, and therefore, manual work is required not only when setting or adding a new base station apparatus, but also when modifying the route of the optical fiber or repairing degradation due to elapse of time. Burden and cost of maintenance and control operations increase.
In JP 2002-118870A, the quantity of delay is measured by test radiation of radio waves in the air. However, since the communication environment changes every moment, the main path of the test wave may happen to be blocked by obstacle. Accordingly, the delay cannot always be measure at high precision. The radio wave transmitted from and received at a mobile terminal requires only such a power level so as to propagate between the mobile terminal and the antenna unit connected to the base station apparatus. In contrast, the test wave emitted from an antenna unit is strong enough to propagate to another antenna unit located in the adjacent cell. Accordingly, the test wave becomes a large interfering wave for the radio signal transmitted from the mobile terminal.
Returning to FIG. 1, one of the baseband processors 114 is provided for each antenna unit 104 by one-to-one correspondence. Accordingly, when increasing the number of sectors, additional antenna units 104 and baseband processors 114 have to be provided. This arrangement is suitable to such applications when the number of users accommodated in the system is increased by increasing the resources of the antenna units 104 and baseband processors 114. However, this arrangement is unsuitable to an application of increasing the area size of each baseband processor, that is, an application where the number of the antenna units 104 is increased to expand the communicating space, while maintaining the number of users accommodated by the system (or the overall sectors). This is because increasing the number of antenna units 104 to expand the communicating space results in excessive resources and processing capacity. Such application includes communications in mountain areas, sparsely populated regions, underground malls, and inside tunnels or buildings.
FIG. 2 illustrates the above-described application to increase the area size per baseband processor. In this example, at most a hundred people are likely to conduct wireless communication inside the five-story building. Each baseband processor 114 is capable of performing baseband processing for signal transmission of a hundred users. To allow wireless communications at all the floors, an antenna unit 104 has to be provided to each floor. In this conventional case, the base station apparatus 110 located in the basement has to be furnished with five baseband processors 114, which is excessive for a hundred users accommodated in this building.